1. Field of the Invention
The present invention relates to the corrosion protection of the critical parts of offshore marine steel structures. Specifically, in the present invention the critical parts such as the nodes are made electrochemically noble or passive. The noncritical parts of the offshore steel structure remain active. Because the active and passive areas of the structure are both electronically and electrolytically coupled, cathodic protection of the critical parts occurs.
2. Description of the Prior Art
In the prior art, corrosion protection of the critical components of offshore platforms, such as nodes, is generally accomplished by cathodic protection of the entire underwater jacket area. This protection is usually with sacrificial anodes such as aluminum, but occasionally impressed current systems are used. In either event current is supplied from the cathodic protection anodes to the entire underwater steel jacket area of which the nodes are an integral part. Since the critical platform parts, such as the nodes, normally constitute only approximately 15% of the underwater jacket area, the existing cathodic protection designs waste about 85% of the cathodic protection current capacity on noncritical jacket members such as platform legs or tubular braces.
It is generally accepted that the structurally critical areas, such as the nodes, cannot tolerate the initiation of corrosion pitting since such pits act as stress raisers. Stress raisers can initiate stress corrosion cracking (SCC), corrosion fatigue or, with cyclical action, mechanical fatigue. Any of these mechanisms can result in catastrophic failure of the critical component. Platform structural damage or even collapse could result. It is also generally accepted that a great amount of corrosion is acceptable on the noncritical steel members of the platform jacket such as the legs and tubular bracing. The prime purpose of conventional aluminum anodes on offshore jackets is to "throw" cathodic protection current into the "hidden" recesses of a critical platform component, such as the node.
Since welding is not acceptable on heat treated nodes, the usual design for aluminum anodes involves a clustering of anodes as close as possible to the "hidden" recesses of the node by welding the anodes on the tubular braces adjacent to the node. This may entail the installation of 50, 100 or even 200% more aluminum anodes than would be required if rapid corrosion protection of the structure were not required. The geometry of some complex nodes makes it impossible to achieve either instantaneous or even adequate cathodic protection over a period of time. Another example of a difficult to protect area is a surface casing within a guide cone. When a large number of wells are drilled in a cluster, it is impossible to provide adequate cathodic protection to the surface casings in the center of the well bay.
The above are examples of circumstances under which the existing cathodic protection systems cannot provide adequate corrosion control to either the platform or the well casing. It must be understood that economic stakes are enormous. Some of the largest deep water platform jackets cost over $100 million. Some completed production platforms with wells cost over $1 billion. The ultimate value of the reservoir produced per billion dollar platform would likely exceed $10 billion. It is, therefore, imperative from an economic, environmental, governmental mandate or insurance requirement that the most effective corrosion control program possible be utilized. In addition to the above, the following are specific problems faced with prior art cathodic protection systems.
High strength steels are more susceptible to hydrogen embrittlement than are the mild steels more commonly used in offshore marine platform jacket construction. Hydrogen embrittlement can result in catastrophic failure of high strength steel. The more advanced design concepts for deep water, such as the Tensioned Leg Platform (TLP) or Guyed Tower, employ large quantities of high strength steel. Cathodic protection, even using aluminum anodes, can produce potentials sufficiently electronegative (e.g., more negative than -1.1 volt to a copper sulfate electrode) to initiate hydrogen embrittlement. The prior art systems, therefore, pose a potential problem to high strength steel.
Conventional aluminum anode cathodic protection designs are structurally incompatible with the rigors and stresses imposed during the launching operation and pile driving on platform jackets. Contact with a launching tether can dislodge the anodes. In early pile driving tests, sacrificial anodes fell off the jacket in large quantities, often removing large segments of platform structural members to which they were welded. In subsequent designs, the aluminum anodes had to be relocated to areas where the mechanical stresses from pile driving were minimized. Unfortunately, this often precludes locating anodes where they are required to optimize current distribution, for the prevention of corrosion in the "hidden" areas such as the reentrant angles of nodes.
Sacrificial anodes endanger diver and Remote Operated Vehicle (ROV) inspections since they obstruct the natural passageways through the tubulars on the underwater jackets. The anode projections obstruct freedom of movement and endanger the umbilicals of divers or ROV's. Even after the sacrificial anodes have been consumed, their core steel reinforcements remain. These anode remnants pose a life-threatening hazard to diver operations. If an impressed current cathodic protection system is used, it must be turned off to avoid diver electrocution.
Sacrificial anodes are heavy, adding approximately 6% to the platform jacket weight. This weight creates additional stress on the jacket, especially during the launching and uprighting operations. Even more important than weight, however, is the large cross sectional area of a sacrificial aluminum anode design. When acted on by ocean currents, the anode area creates a lateral force on the platform jacket. Lateral wave force acting on the anodes increases the overturning moment on the marine structure. This incremental overturning moment must either be compensated for (for example, by additional piles) or the designer must accept a reduced safety margin on the platform jacket.
Cathodic protection systems are a nuisance in well logging and similar data logging activities. The direct currents encountered from conventional cathodic protection systems must be "screened out" in order to interpret the logs. Impressed current cathodic protection systems are turned off before logging. It is not possible to turn off sacrificial anodes like aluminum when they are welded to the jacket.
Calcareous deposits (principally CaCO.sub.3 and Mg(OH).sub.2) result from cathodic protection of steel in sea water. These deposits are generally beneficial in reducing cathodic protection current requirements over a period of time. However, these same calcareous deposits must be mechanically removed by divers during inspection for defects (e.g., pits or cracks) on critical areas. Such inspections are routinely performed by the operating companies and in some cases are mandated by regulatory bodies or insurance requirements.
Most designers of cathodic protection systems will add a safety factor of from 10% to as much as 100% dependent on the platform location and previous experience at that location. While safety factors cost money, experience has shown that underdesign is a more expensive alternative since underwater retrofit typically costs 10 to 100 times (dependent on water depth) the original installation cost.
Sacrificial and impressed current anodes are subject to variable behavior dependent on their environment and composition. Seawater temperature, pressure (depth), salinity, and water velocity may have a profound effect upon anode output, efficiency and life expectancy. It can, therefore, be seen that existing cathodic protection systems leave much to be desired.
In addition to the problems described above with existing cathodic protection systems, there is a serious problem with the removal of offshore marine structures. In the North Sea alone, the eventual cost of removal of 139 structures is estimated to be over ten billion dollars. In the Gulf of Mexico, over 3,000 existing structures eventually will require removal. In the prior art, the smaller offshore platform jackets weighing less than 3000 tons are generally removed by the use of large amounts of explosives which sever the jacket piles below the mud line. The platforms are then either floated or placed on barges and brought ashore. With this explosive process, the marine life, in particular fish and turtles which always thrive around offshore platforms, is destroyed.
For platform jackets weighing more than about 3000 tons, the existing alternatives for removal are to topple the platforms in situ with explosives or to provide navigational lights and to let seawater corrosion eventually topple and destroy the platform. Explosives destroy marine life and toppling poses a hazard to navigation since the base of a platform after toppling may protrude 300 feet or more.
Fishermen's nets, lines and trawler boards can become entangled with a toppled platform. Seawater corrosion at an average corrosion rate of 5 mils per year (0.005 inches) will likely take 100 to 300 years to topple most platforms. In the meantime, the platforms present a constant danger to navigation and to unauthorized personnel who trespass.
Consequently, an improved solution is required for the destruction of offshore structures whose useful life is over. The need is especially acute for destruction of deep water structures weighing over 3000 tons.